1. Field of the Invention
This invention relates to an image forming apparatus of a type which transfers a toner image formed on an image bearing member to a transfer material through an intermediate transferring member.
2. Related Background Art
In a color image forming apparatus of the electrophotographic type (for example, a copying apparatus or a laser beam printer), so-called primary transferring in which a toner image formed on a photosensitive drum as an image bearing member is once transferred onto an intermediate transferring member and is repeated a plurality of times to thereby superpose toner images of plural colors on the intermediate transferring member, and these toner images of plural colors are collectively secondary-transferred onto a transfer material such as paper.
FIG. 13 of the accompanying drawings shows an example of a color image forming apparatus using an intermediate transferring belt as the intermediate transferring member.
The image forming apparatus shown in FIG. 13 is provided with a photosensitive drum 101, and around the photosensitive drum 101 supported for rotation in the direction of arrow R1, there are disposed four developing devices 105, 106, 107 and 108 in which black (Bk) toner, cyan (C) toner, magenta (M) toner and yellow (Y) toner are contained respectively. One of these developing devices which is used for the development of an electrostatic latent image on the photosensitive drum 101 is designed to abut against the photosensitive drum 101 by means (not shown) for moving it toward and away from the photosensitive drum.
The photosensitive drum 101 is uniformly charged by a charger 102, and an electrostatic latent image is formed thereon by a laser beam (scanning light) 104 from a laser exposure optical system 103. Next, the electrostatic latent image is developed as toner images with the toners caused to adhere thereto by the developing devices 105-108, and in a primary transfer nip portion N.sub.1, the toner images are primary-transferred onto an intermediate transferring belt 109 by a primary transferring roller 110. The formation, development and primary transferring of the electrostatic latent image are successively effected in the order of Y, M, C and Bk with respect to the four colors by the developing devices 108, 107, 106 and 105, whereby toner images of four colors superposed one upon another are formed on the intermediate transferring belt 109. These toner images are then collectively secondary-transferred onto a transfer material 118 conveyed while being held by and between a secondary transferring roller 111 and the intermediate transferring belt 109.
The primary transfer and the secondary transfer will further be described in detail. First, when the photosensitive drum 101 is, for example, an OPC (organic photo-semiconductive) photosensitive member having a negative charged characteristic, toners of negative polarity are used to develop the exposed portion to which the laser beam 104 has been applied by the developing devices 108, 107, 106 and 105. Accordingly, a primary transfer bias of positive polarity is applied to the primary transferring roller 110 by a primary bias voltage source 120.
Here, as the intermediate transferring belt 109, use can be made, for example, an endless resin belt having a thickness of the order of 100 to 300 .mu.m and resistance-adjusted to volume resistivity of the order of 10.sup.11 to 10.sup.16 .OMEGA..cm. In this case, as the material of the resin belt, use can be made of resin film (resistance-adjusted as required) such as PVdF (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate) or polycarbonate. Also, as another example, there is a case where the above-described resin belt is adjusted to a volume resistance value of the order of 10.sup.7 to 10.sup.11 .OMEGA..cm by carbon, ZnO.sub.2, SnO.sub.2, TiO.sub.2 or other electrically conductive filler and used. By achieving medium to low resistance like the latter, it is possible to prevent the formation of a bad image by charges being accumulated in the intermediate transferring belt 109.
As still another example, a rubber material (such as chloroprene rubber, EPDM, NBR or urethane rubber) having lower hardness than resin and having a thickness of the order of 0.5 to 2 mm and adjusted to volume resistivity of the order of 10.sup.6 to 10.sup.11 .OMEGA..cm can be used as the material of the intermediate transferring belt 109.
The intermediate transferring belt 109 is passed over a back roller 112, a drive roller 115, and a tension roller 116. As the primary transferring roller 110, use is usually made of a low resistance roller having volume resistivity of 10.sup.5 .OMEGA..cm or less. In the foregoing, the primary transferring roller 110 and the primary bias voltage source 120 together constitute primary transferring means.
Next, the secondary transferring of the toner images to the transfer material 118 is effected by secondary transferring means constituted by the secondary transferring roller 111, the back roller 112, the secondary bias voltage source 121. The secondary transfer is effected by disposing the back roller 112 of low resistance grounded or having a suitable bias applied thereto as an opposing electrode inside the intermediate transferring belt 109, holding the intermediate transferring belt 109 between the back roller and the secondary transferring roller 111 of low resistance disposed outside the intermediate transferring belt to thereby constitute a secondary transferring region N.sub.2, applying a secondary transfer bias of positive polarity to the secondary transferring roller 111 by the secondary bias voltage source 121, and causing the secondary transferring roller 111 to bear against the transfer material 118 from the back side thereof.
After the above-described primary transferring has been completed, the photosensitive drum 101 has any residual toner of primary transferring remaining on its surface removed by a cleaner 119, and has any residual charges thereon removed by an exposure device 117, and is used for the formation of the next toner images.
On the other hand, after the above-described secondary transferring has been completed, the intermediate transferring belt 109 has any residual toner of secondary transferring remaining thereon removed by a cleaner 113, whereafter it has its charges removed by a charge removing charger (charge removing means) 114. AC corona charging is often used as the charge removing charger 114.
The charge removing charger 114 can be omitted when the intermediate transferring belt of low to medium resistance as previously described is used.
As the above-described intermediate transferring member, there is available an intermediate transferring drum besides the intermediate transferring belt 109, but generally the intermediate transferring belt 109 is excellent in the height of the degree of freedom of the disposition of the intermediate transferring member and the good separability of the transfer material 118 after the secondary transferring (the possibility of the separation of curvature in the bent portion of the belt), as compared with the intermediate transferring drum. In contrast, the intermediate transferring drum can be simplified in structure, as compared with a case where the belt is driven. Even in the case of the intermediate transferring drum, the electrical characteristic of a resin or rubber layer provided on the surface of an electrically conductive cylindrical member can be considered as in the case of the intermediate transferring belt 109 and therefore, detailed description will be omitted.
In the above-described image forming apparatus, the order of formation of the toner images of four colors (Y, M, C and Bk) (the order of primary transfer) has heretofore been such that toner images are formed in suitable order by the toners of three colors, i.e., Y toner, M toner and C toner, and finally a toner image is formed by Bk toner.
Also, instead of the cleaner 113 and charge removing charger 114 of FIG. 13, a charging roller 122 movable toward and away from the intermediate transferring belt 109 has heretofore been provided as shown in the image forming apparatus of FIG. 14 of the accompanying drawings. In FIG. 14, the construction is the same as that of FIG. 13 except for the charging roller 122, and like members are designated by like reference numerals. The charging roller 122 secondary-transfers the toner images of the intermediate transferring belt 109 to a transfer material, and thereafter charges any residual toner of secondary transferring remaining on the intermediate transferring belt 109 to a polarity opposite to the regular charging polarity of the toners in the developing devices. When an image is to be continuously formed, the next Y toner image on the photosensitive drum is primary-transferred to the intermediate transferring belt in the primary transfer nip portion by the primary transferring roller 110 and simultaneously therewith, the residual toner of secondary transferring charged by the charging roller 122 is transferred from the intermediate transferring belt 109 to the photosensitive drum 101. By doing so, the throughput of image formation has heretofore been improved.
However, the above-described example of the prior art has suffered from the following problem. Color characters and color lines (e.g. red characters and red lines=Y toner+M toner) of plural colors superposed one upon another formed on the intermediate transferring belt 109 by the use of color toners, i.e., Y toner, M toner and C toner and toners not yet formed on the intermediate transferring belt 109 (e.g. C toner and Bk toner) have been gradually scattered while the intermediate transferring belt 109 is rotated to form these toners on the intermediate transferring belt 109. Thereby, the hue or tone of the color characters and color lines has been changed and the resultant image has become bad. This is more liable to occur as the resistance of the intermediate transferring belt 109 becomes lower, and has been a remarkable problem in a belt type wherein as the intermediate transferring member, the intermediate transferring belt 109 as described above is bent by the inside rollers 112, 115 and 116. In contrast, it is possible to decrease the amounts of toners of respective colors (Y toner, M toner and C toner) to thereby improve the scatter thereof, but if this is done, another problem that the resultant image becomes light in color will arise.
Also, in the above-described image forming apparatus of FIG. 13, the order of the colors in the primary transferring is Y.fwdarw.M.fwdarw.C.fwdarw.Bk, i.e., Y toner which is a color toner (Y toner, M toner or C toner) as the first color and therefore, on this side of the vicinity of the nip formed in the primary transfer nip portion N.sub.1 by the photosensitive drum 101 and the intermediate transferring belt 109 (the upstream side with respect to the direction of rotation of the intermediate transferring belt 109), the discharge of electricity in air occurs between the photosensitive drum 101 and the intermediate transferring belt 109 and as the result there has arisen the problem that the intermediate transferring belt 109 and the residual toner of secondary transferring on the intermediate transferring belt 109 is charged to the minus polarity immediately before primary transferring and therefore the residual toner of secondary transferring is not collected by the photosensitive drum 101.